(1) Field of the Invention
The present invention relates to a dry developer for an electrostatic image and a process for the preparation thereof. More particularly, the present invention relates to a molded particulate developer consisting of a dispersion comprising a binder resin, a wax and a pigment, which is improved in the dispersibility of each component and the homogeneity of the composition.
(2) Description of the Prior Art
A product obtained by dispersing a pigment such as a coloring pigment, an extender pigment, a magnetic pigment or a conductive pigment in a binder medium which shows a fixing property under application of a heat or pressure and molding the dispersion into particles is widely used as a dry developer for an electrostatic image. As the fixing binder medium, resins are ordinarily used, but if resins alone are used, the fixing property is often insufficient, and therefore, waxes are widely used in combination with the resins. For example, in the case where developer particles are pressure-fixed, the resins are too hard and hence, it is necessary to use soft components such as waxes in combination with the resins. Furthermore, in the case where developer particles are fixed by contact with a heating roll, in order to prevent an offset phenomenon in which the developer particles migrate to the surface of the heating roll, it is desirable to use a wax as a parting agent in combination with a resin binder.
In a dispersion system comprising a binder resin, a wax and a pigment, it is very difficult to disperse the respective components uniformly and homogeneously. More specifically, waxes are oleophilic and non-polar, while most of pigments are hydrophilic and polar. Accordingly, if a pigment and a wax are dispersed in the molten state in a resin having a good dispersibility to pigments, it is found that the wax is heterogeneously dispersed in the form of considerably coarse particles, and if this molten dispersion is pulverized after cooling or under cooling to form particles, the composition greatly differs among respective particles and the resulting particulate developer is significantly insufficient in the adaptability to the developing and fixing operations and also in the quality of the formed image. For example, among these developer particles, those having a very high wax content cause reduction of the heat resistance of the developer and decrease of the flowability of the developer. Moreover, particles having an extremely low wax content cause an offset phenomenon on the heating roller. Moreover, for example, in case of a one-component type magnetic developer, since development is carried out based on the balance between the Coulomb force and the magnetic attractive force, developer particles having a low magnetic pigment content adhere to the background, resulting in increase of the fog density.
In addition to the foregoing defects, there arise still other problems. More specifically, when a heterogeneously dispersed molten mixture is pulverized after cooling or during cooling, heterogeneous portions of the texture are readily crumbled to form fine particles, which are removed at the classification step. Therefore, the yield of the particulate developer is reduced. Furthermore, in case of a two-component type developer, such fine particles cause contamination of a carrier.